Method of disconnecting a short-circuited part between mutually opposed electrodes

ABSTRACT

A method of disconnecting a short-circuited part between upper and lower electrodes is provided which comprises the steps of; 
     (a) forming a cell by placing one upon another a first substrate provided with a common electrode and a second substrate provided with opposed electrodes in the manner that said common electrode and said opposed electrode may form a plurality of mutually opposed portions; 
     (b) detecting, among the plurality of mutually opposed portions, a mutually opposed portion at which a short circuit has occurred; and 
     (c) disconnecting by insulation a short-circuited site present at the mutually opposed portion from the common electrode or opposed electrode.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a method of repairing a liquid crystaldisplay panel, and more particularly to a method of repairing aferroelectric liquid crystal display panel, that repairs a short circuitoccurred between upper and lower electrodes owing to inclusion of aforeign matter in a cell.

2. Related Background Art

Since publication of twisted-nematic liquid crystals (TN liquidcrystals) in "Applied Physics Letters", Vol. 18, No. 4, pp.127-128, byShadt and W. Helfrich in 1971, liquid crystal display panels have maderapid progress, but recent trend in development of goods is shiftingfrom original numerical display panels using segment electrodes tocharacter display panels using a dot matrix. Particularly in recentyears demanded is commercialization of liquid crystal display panelshaving a picture element of a large capacity and capable of answeringfor the display screens of word processors or personal computers and thedisplay screens of televisions.

However, in multiplexing drive of the above TN liquid crystals, dutyratio decreases in the proportion of 1/N with increase in the number (N)of scanning lines, and hence there has been a limit in increasing thenumber of scanning lines. In the case of liquid crystal display panelsof the type that picture elements formed at the sites at which scanninglines and data lines cross are controlled for each picture element, thinfilm transistors in the number corresponding to the number of pictureelements are required to be formed on a liquid crystal display panelsubstrate, but a technically very difficult problem has been involved informing this thin film transistors over a large area.

As a liquid crystal display panel that can solve these problems,proposed was the surface s&ability ferroelectric liquid crystal devicepublished in "Applied Physics Letters", Vol. 36, No. 11, pp.899-901, byN. Clark and S.T Lagerwall in 1980. This ferroelectric liquid crystaldevice produces two stably aligned states by which a contrast can bediscriminated according the direction of an electric field. This hasmade it necessary to see the cell thickness to be small enough to bringthe spiral structure inherent in a chiral smectic liquid crystal todisappear, which thickness is exemplified by 0.5 μm to 2 μm inapproximation.

For &his reason, there have been the problem that a short circuit mayoccur between upper and lower electrodes simply by inclusion of a minuteforeign matter in the inside of a cell of the above ferroelectric liquidcrystals. In particular, when the ferroelectric liquid crystal cellwired as the upper and lower electrodes with matrix electrodes (ascanning electrode and a signal electrode) for the multiplexing drive,any short-circuited part present at the site at which scanning lines anddata lines cross may be identified by a vieWer as a line defect(non-switching line) having a shape of a cross formed by the scanningline and signal line at the short-circuited part, resulting in aloWering of display quality.

SUMMARY OF THE INVENTION

Accordingly, an object of the present invention is to provide a methodof repairing a liquid crystal display panel that can solve the aboveproblems, and to provide a liquid crystal display panel whose displayquality has been thereby improved.

Another object of the present invention is to provide a process ofpreparing a liquid crystal display panel that can achieve an improvedyield in the manufacture of liquid crystal display panels.

According to an aspect of the present invention, there is provided amethod of disconnecting a short-circuited part between upper and lowerelectrodes, comprising the steps of;

(a) forming a cell by placing one upon another a first substrateprovided with a common electrode and a second substrate provided with anopposed electrode in the manner that said common electrode and saidopposed electrode may form a plurality of mutually opposed portions;

(b) detecting, among said plurality of mutually opposed portions, aportion at which a short circuit has occurred; and

(c) disconnecting by insulation a short-circuited o site present at themutually opposed portion from said common electrode or opposed electrodeand a process of preparing a liquid crystal display panel by using thesame.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is a plan view illustrating a display panel having been notrepaired, FIG. 1B is an enlarged plan view of a short-circuited sitethereof, and FIG. 1C is a cross section along the line A-A';

FIG. 2 is a block diagram of a repair apparatus used in the method ofthe present invention;

FIG. 3A is a plan view illustrating the method of the present invention,and FIG. 3B is a cross section thereof;

FIG. 4A is a plan view illustrating a display panel having been repairedaccording to the present invention, and FIG. 4B is a cross sectionthereof;

FIG. 5A is an electron microscope photograph showing a state of metalstructure on an upper substrate after repairment, and FIG. 5B is anelectron microscope photograph showing a state of metal structure on alower substrate after that.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

The present invention will be described below with reference to theaccompanying drawings.

FIG. 1A is a plan view illustrating a liquid crystal display panel 11,and FIG. 1B is an enlarged plan view of the part X in FIG. 1A. FIG. 1Cis a cross section along the line A-A' in FIG. 1B.

The liquid crystal display panel 11 illustrated in FIG. 1 is providedwith ferroelectric liquid crystal 16 between an upper substrate 13provided with a transparent common electrode (a terminal) 12 (thatfunctions, for example, as a scanning electrode at the time ofmultiplexing drive) and a lower substrate 15 provided with transparentopposed electrodes (a terminal) 14 (that functions, for example, as asignal electrode at the time of multiplexing drive). In the liquidcrystal display panel 11 illustrated in FIG. i, a non switching scanningline α and a non-switching signal line β is identified in a displayscreen by a viewer. Line N in FIG. 1B corresponds to a line on which nomutually opposed electrodes are formed. The position of a polarizationaxis of a polarizer and an analyzer is so set that the liquid crystalalignment on the line N may be in the dark state when the liquid crystalis placed between the polarizer and the analyzer. The transparent commonelectrode 12 and the transparent opposed electrodes 14 are provided withelectrically connected low-resistance lines 17b and 17a, respectively.This low-resistance lines 17b and 17a can be formed using metals such assilver, copper, gold, aluminum, chromium and molybdenum or alloys of anyof these and may have a film thickness in the range of from 500angstroms to 5,000 angstroms, preferably from 1,000 angstroms to 3,000angstroms. The line width thereof may preferably be set in the range offrom 10 μm to 50 μm.

The defective line α-β mentioned above which appeared in the displayscreen, is a site corresponding to a crossing part of the defective lineα-β and caused by a short circuit occurred between a transparent commonelectrode 12a and a transparent signal electrode 14a as illustrated inFIG. 1C. AT this short-circuited site, a foreign matter 18 asexemplified by dust and resin is included between the lower substrate 15and transparent opposed electrode 14a, thus producing a convex on thetransparent opposed electrode 14a, which causes a short circuit incontact with the transparent common electrode 12a. Besides the abovestate, the causes of the short circuit may also include a state in whicha conductive particle is held between the transparent common electrode12 and transparent opposed electrode 14.

Such a short-circuited part can be detected by actuating terminals 19aand 19b of a circuit tester 19.

FIG. 2 is a block diagram illustrating a repair apparatus used in themethod of the present invention. In the figure the numeral 21 denotes amicroscope; 22, a viewer; 23, an objective lens; 24, a slit; 25, a pilotlight source: 26, a beam expander; 27, a laser beam; and 28, a laserbeam generator.

In the repair apparatus illustrated in FIG. 2, &he short-circuited si&eexisting in the ferroelectric liquid crystal display panel 11 isdetected by the microscope 21, and thereafter the positioning of thesite to which the laser beam is irradiated and the spot-setting of thelaser beam 27 can be carried out by irradiating light from the pilotlight source 28. On the light path from the pilot light source 25 to theferroelectric liquid crystal display panel 11, provided are half mirrors20a and 20b, and a slit 24 that controls the X-axis and Y-axis of thelaser-beam irradiated part 31 and narrows the beam width.

Next, the laser beam 27 from the laser beam generator 28 is, after beingexpanded in its width through the beam expander 26, reflected by halfmirrors 20b and 20a, and irradiated on the ferroelectric liquid crystaldisplay panel. Thus, in the present invention, irradiating the laserbeam 27 as illustrated in FIG. 3 (a laser beam irradiating part 31)enables formation of an insulation disconnected part 41 at which theshort-circuited site is disconnected from the transparent commonelectrode 12a and transparent opposed electrode 14a as illustrated inFIG. 4.

In the present invention, used as the laser beam 27 are a YAG laserbeam, a ruby laser beam and a CO₂ laser beam, and the YAG laser(oscillation wavelength: 1.06 μm) may preferably be used in instances inwhich the transparent&electrodes 12 and 14 are formed with ITO (indiumtin-oxide) and also the low resistance lines 17a and 17b are formed withmolybdenum. The beam spot of this laser beam 27 can be arbitrarily set,but may generally range from 1 μm to 100 μm, preferably from 1 μm to 10μm in approximation. Laser power may be from 0.1 mJ/pulse to 10 mJ/pulsein approximation, and its pulse width may be from 1 to 100 nsec inapproximation. In this occasion the insulation- disconnected part 41 isrequired to be formed so that the transparent common electrode 12 and&he transparent opposed electrode 14 may not undergo any insulationdisconnection from their respective terminals.

A step of forming the above insulation-disconnected part 41 can beintroduced after a step of placing one upon another the upper substrate13 provided with the transparent common electrode 12 and the lowersubstrate 15 provided With the transparent opposed electrode 14 to forman empty cell, and before a step anterior to a step of filling saidempty cell with liquid crystals. The step of forming the insulationdisconnected part 41 can also be introduced in a step posterior to thestep of filling the empty cell with liquid crystals. Here, in instancesin which the liquid crystals in the cell comprises ferroelectric liquidcrystals, the liquid crystals are locally heated at the part onto whichthe laser beam 27 is irradiated, causing alignment disturbance at thispart, so that there may preferably be employed a re-alignment step. Morespecifically, the re-alignment of liquid crystals 16 can be achieved byheating, after irradiation of the laser beam 27, the liquid crystals 16filled in the ferroelectric liquid crystal display panel 11 until theycome to an isotropic phase and thereafter cooling it gradually at therate of about 5° C/h.

The upper substrate 13 and/or lower substrate 15 used in the presentinvention may preferably be provided with an alignment& control filmwhich has been subjected to monoaxial alignment treatment. The monoaxialalignment treatment can be conducted by rubbing, oblique evaporation oroblique etching. Usable as the alignment control film are, for example,organic alignment control films made of polyimide, polyamide,polyamidoimide, polyester, polyester imide, polyvinyl alcohol,polyethylen, etc. or inorganic alignment control films made of SiO,SiO₂, TiO₂, etc. disclosed in U.S. Pat. Nos. 4,561,726 and No.4,639,089.

Experiments made by the present inventors also revealed that, even afterthe local heating by the laser beam 27, the function of alignmentcontrol is little adversely affected when polyimide, polyamidoimide,polyester imide, polyamide, polyvinyl alcohol or SiO₂ is sued as thealignment control film.

In a preferred embodiment of the present invention, a highly dielectricinsulating film formed with SiO₂, TiO₂ or Ta₂ O₅ can be provided betweenthe common electrode and alignment control film and between the opposedelectrode and alignment control film. This insulating film is formed tohave a film thickness of generally from 1OO angstroms to 5,000angstroms, preferably from 300 angstroms to 2,000 angstroms. Here, if aconductive foreign matter is included between the common electrodesubstrate and opposed electrode substrate, this conductive foreignmatter may break through the insulating film formed on the electrode tocause a short circuit between the common electrode and opposedelectrode. Accordingly, the repairing method previously described may beused to eliminate the appearance of the defective line α-β asillustrated in FIG. 1A.

In the present invention, usable as the above ferroelectric liquidcrystals 16 are, for example, those disclosed in U.S. Pat. No.4,561,726, No. 4,589,996. No. 4,592,858, No. 4,596,667, No. 4,613,209,No. 4,614,609, No. 4,615,586, No. 4,622,I65, No. 4,639,089, etc.

In the present invention, it is also possible to provide a spacer memberfor making control to keep constant the space between the uppersubstrate 13 and lower substrate 15. Usable as this spacer member areplastic beads, glass beads or the like having an average particlediameter of from 0.5 μm to 5 μm. Here, usable as the plastic beads areadhesive resin beads enabling adhesion between the upper substrate 13and lower substrate 15. There can be also used mixed beads comprisingthis adhesive resin beads and the glass beads.

The present invention will be described below by giving Examples.

EXAMPLE 1

On glass substrates, stripe-shaped electrodes o respectively comprisingindium oxide and molybdenum were formed in the manner as shown in thefollowing table.

    ______________________________________                                                  Indium oxide                                                                            Molybdenum                                                          (Transparent                                                                            (Low-resistance                                                                            Space                                                  electrode)                                                                              line)        be-                                                      Elec-   Film    Elec-  Film  tween                                            trode   thick-  trode  thick-                                                                              elec-                                Number      width   ness    width  ness  trodes                               ______________________________________                                        Upper  400                                                                    electrode                                                                            lines                                                                  Lower  640      280     1,000 20     2,000 20                                 electrode                                                                            lines    μm   Å μm  Å μm                              ______________________________________                                    

Next, a polyimide polymeric film (using SP-710; trade name; availablefrom Toray lndustries, lnc.) was provided on the electrode substrates,and rubbing alignment treatment was carried out in one direction byusing cloth. On one of &he substrates, an isopropyl alcohol solutioncontaining 0.05% by weight of SiO₂ beads having an average particlediameter of 1.5 microns was further spin coated, followed by heat dryingand thereafter placing the upper substrate on the lower substrates withthe interposition of a seal material comprising an epoxy adhesive, andthen heating under contact bonding to form a panel (empty cell) having agap of about 1.5 microns. In the panel thus obtained, ferroelectricliquid crystals (CS-1014: available from Chisso Corporation) wereenclosed to prepare a display device. Subsequently application ofdisplay signals between the upper and lower electrodes resulted inappearance of a part that showed a display in a state different fromother electrode portions, in the shape of a cross as illustrated in FIG.1A, and measuring the resistance value between the upper and lowerelectrodes corresponding to that part by use of a circuit testerrevealed that there was shown about 30 K.ohms and was seen ashort-circuited state. Observation of the corresponding picture imagewith use of a microscope (magnification: 50) shows a foreign matter ofabout 10 μm in diameter in the picture element.

This display device was set in a YAG laser beam repair apparatus(SL456B; available from NEC) illustrated in FIG. 2, and the electrodewas cut on the periphery of the foreign matter by the laser beam, underthe following conditions and in the state that the display device standsas it is.

Wavelength: 1.06 μm

Power: 2 mJ/pulse

Beam: 5 μm=10 μm

Pulse width: 20 nsec

The resulting states are shown in FIG. 5. Measuring again the resistancevalue between the same electrodes with use of a tester reveled that thevalue was 20 M.ohms or more, which was equal to the resistance valueshown in normal electrodes. This display device was further subjected toheating for 2 hours at a temperature of 90° C and gradual cooling for 8hours to a temperature of 40° C, followed by application of displaysignals between the upper and lower electrodes. As a result, there wasseen no line defect in the shape of a cross illustrated in FIG. 1A, and,also in the picture element remedied by the laser repair, there wasshown switching characteristics in the state that liquid crystals werealigned without any problem in image quality to the naked eye, thusbringing about remarkable improvement in the display quality.

FIG. 5A shows a state of the upper substrate after the upper substrateand lower substrate of the empty cell were peeled off, and FIG. 5B showsa state of the lower substrate thus peeled, each of which is amicroscope photograph of 1,000 magnification.

EXAMPLE 2

Example 1 was exactly repeated to carry out an experiment, except thatthe step of insulation disconnection treatment on the short circuitedsite with use of the YAG laser beam repair apparatus used in Example 1was carried out in a step before the empty cell was filled withferroelectric liquids. As a result, there were obtained the same resultsas in Example 1.

EXAMPLE 3

Example 1 was exactly repeated &o carry out an experiment, except thatthe polyimide film used when the cell of Example 1 was prepared wasreplaced by a polyvinyl alcohol film. As a result, there were obtainedthe same results as in Example 1.

EXAMPLE 4

Example 1 was exactly repeated to carry out an experiment, except thatan Si0₂ film of 500 angstrom thick was previously formed before thepolyimide film was formed on the glass substrate used when the cell ofExample 1 was prepared As a result, there were obtained the same resultsas in Example 1.

EXAMPLE 5

Example 4 was exactly repeated to carry out an experiment, except that aTa₂ O₅ film Was used in place of the SiO₂ film used in Example 4. As aresult, there were obtained the same results as in Example 4.

EXAMPLE 6

Example 4 was exactly repeated to carry out an experiment, except that aTiO₂ film was used in place of the SiO₂ film used in Example 4. As aresult there were obtained the same results as in Example 4.

We claim:
 1. A method of disconnecting a short-circuited partcomprising: the steps of:providing a first substrate comprising aplurality of upper side electrodes and a second substrate comprising aplurality of lower side electrode, wherein said first and secondsubstrates each have alignment control films subjected to a monoaxialalignment treatment; orienting said first and second substrates toprovide a gap thereinbetween such that said upper and lower electrodesform a plurality of mutually opposed portions; detecting whether or nota short-circuit has occurred among said plurality of mutually opposedportions; and disconnecting the short-circuited site present at themutually opposed portion from at least one of said upper side of saidlower side electrodes.
 2. The method of disconnecting a short-circuitedpart according to claim 1, wherein said disconnecting step comprisesirradiating a laser beam.
 3. The method of disconnecting ashort-circuited part according to claim 2, wherein said laser beam is aYAG, a ruby or a CO₂ laser beam.
 4. The method of disconnecting ashort-circuited part according to claim 1, wherein said short-circuitedsite is disconnected by insulation from the upper side electrode and thelower side electrode.
 5. The method of disconnecting a short-circuitedpart according to claim 1, wherein said disconnecting step comprisesinsulating the short-circuited site present at the mutually opposedportion from said upper side electrode and lower side electrode, suchthat the electrical integrity of the upper and lower side electrodesfrom which the short-circuited site is disconnected is maintained aftersuch disconnection.
 6. The method of disconnecting a short-circuitedpart according to claim 1, wherein the plurality of said upper sideelectrodes function either as scanning or signal electrodes and theplurality of said lower side electrodes function as the other.
 7. Themethod of disconnecting a short-circuited part according to claim 1wherein said alignment control film is formed from at least one materialselected from the group consisting of polyimide, polyamidoimide,polyester imide, polyamide, polyvinyl alcohol and SiO₂.
 8. The method ofdisconnecting a short-circuit part according to claim 7, wherein aninsulating film is provided between said alignment control film and saidupper side electrodes, and between said alignment control film and saidlower side electrodes.
 9. The method of disconnecting a short-circuitedpart according to claim 8, wherein said insulating film comprises SiO₂,TiO₂ or Ta₂ O₅.
 10. The method of disconnecting a short-circuited partaccording to claim 1, wherein said monoaxial alignment treating is arubbing treatment.
 11. The method of disconnecting a short-circuitedpart according to claim 1, wherein said monoaxial alignment treatment isan oblique evaporation treatment.
 12. The method of disconnecting ashort-circuited part according to claim 7, wherein said disconnectingstep comprises irradiating a laser beam.
 13. The method of disconnectinga short-circuited part according to claim 12, wherein said laser beam isa YAG, ruby or CO₂ laser beam.
 14. The method of disconnecting ashort-circuited part according to claim 7, wherein said short-circuitedsite is disconnected by insulating the upper side electrode from boththe upper and the lower side electrodes.
 15. The method of disconnectinga short-circuited part according to claim 7, wherein said disconnectingstep comprises insulating the short-circuited site present at themutually opposed portion from the upper side electrode and the lowerside electrode, such that the integrity of the upper and lower sideelectrodes from which the short-circuited site is disconnected ismaintained after such disconnection.
 16. The method of disconnecting ashort-circuited part according to claim 7, wherein a plurality of theupper side electrodes function as scanning electrodes, and a pluralityof the lower side electrodes function as signal electrodes.
 17. Themethod of disconnecting a short-circuited part according to claim 7,wherein an insulating film is provided between said alignment controlfilm and the upper side electrode, and between said alignment controlfilm and the lower side electrodes.
 18. The method of disconnecting ashort-circuited part according to claim 17, wherein said insulating filmcomprises SiO₂, TiO₂ or Ta₂ O₅.
 19. The method of disconnecting ashort-circuited part according to claim 7, wherein the monoaxialalignment treating is a rubbing treatment.
 20. The method ofdisconnecting a short-circuited part according to claim 7, wherein themonoaxial alignment treatment is an oblique evaporation treatment.